It is known to use electronic engine management systems for controlling the operation of an engine within a motor vehicle. It is also known to have a control system for performing "closed loop idle control" to control the idle speed of the engine. This is a control strategy whereby, typically, the actual idle speed of the engine is compared to a desired target value idle speed at regular intervals. The engine idle speed is returned or adjusted to that target value when any deviation from that target value occurs.
In conventional homogeneous charge internal combustion engines, the engine speed and the idle speed in particular are typically controlled by a throttle valve controlling air flow to the engine. In such an engine, the control system will normally define idle as being when the throttle is closed. In contrast, in the case of engines such as the Applicant's stratified charge, air assisted, fuel injected engine, the engine speed is increased or decreased at idle by respectively increasing or decreasing the fuelling rate when under closed loop idle control. Accordingly, in such an engine, the control system may preferably define idle as being when the accelerator pedal of the vehicle is fully disengaged. Typically, the fuelling rate is controlled by varying the opening time, commonly known as the "pulse" time, of the fuel injectors and therefore changing the amount of fuel injected into the cylinders of the engine. The fuel based control system of the Applicant's engine therefore uses the fuelling rate as the primary control parameter and the required air flow is determined as a function of the fuelling rate. Although a throttle valve can and typically will be used to control the air flow to the engine, the throttle valve does not regulate the engine speed as in conventional engines.
In either of these engine applications, idle speed control may occur both when the motor vehicle is stationary and when the motor vehicle is in motion. However, if the control system performs closed loop idle control when the motor vehicle is in motion, significant "driveability problems" associated with the phenomenon commonly referred to as "tip-in/tip-out" can result. This phenomenon can manifest physically as rocking of the engine relative to the motor vehicle chassis during acceleration, such as the throttle opening from idle (tip-in), or deceleration, such as throttle closure to idle (tip-out).
Tip-out problems occur due to the engine being driven through the gearbox during vehicle motion whilst there is no operator demand on the engine, for example, such as when the throttle is closed. In the case of fuelling controlled engines, this tends to maintain the engine speed irrespective of any attempt to control it by fuelling reduction. Accordingly, the closed loop idle control system, sensing this maintained engine speed, typically reduces the fuelling to the engine significantly or totally in an attempt to reduce the engine speed to the target idle speed value. This generally causes a severe vehicle deceleration through the motor vehicle drive-line, which may typically be compounded by induced engine rocking. This is a very undesirable condition which makes the motor vehicle very difficult to drive smoothly.
Tip-in problems in fuelling controlled engines occur partly due to the fuelling rate being significantly reduced (due to closed loop idle) during tip-out as described above. As mentioned, this induces significant motor vehicle deceleration and engine rocking in the forward direction (in a transverse engine configuration) due to motoring through the gearbox. With regard to tip-in, the main problem arises in that, when the operator demand is once again applied, for example, such as the throttle being re-opened, the fuelling level has to be increased very rapidly to get from a significantly low level to the desired level for a drive away or acceleration condition, or too much lag will be felt in the engine response. This however causes a very rapid change in engine torque. This typically results in a reaction force causing the engine to rock rearwards (in a transverse engine configuration) with significant force resulting in a shock through the engine mounts and hence the motor vehicle.
Tip-in problems are compounded further on a motor vehicle having an automatic transmission due to the desired low engine idle speed when there is no operator demand, such as when the motor vehicle is stationary, in order to reduce the torque converter load and to avoid wasting fuel. When a tip-in occurs from such a low idle speed and the change in the fuelling rate is very rapid, the engine speed begins to increase. However, from such a low idle speed, the engine speed has to increase significantly before torque is transferred through the torque converter (or the "stall speed" thereof is reached) thus causing a time delay in response to operator or driver demand for acceleration. As the engine speed increases, the rotating components thereof gather momentum and when decelerated by the torque converter, the energy acquired due to this increasing momentum is dissipated as a severe shock through the motor vehicle drive-line. This is very undesirable and makes the motor vehicle difficult to drive smoothly.
The motor vehicle drive-line will typically include those components that transmit the rotating energy of the engine to the driving wheels of the vehicle. In a vehicle with a transverse engine configuration, the drive-line will include the engine gearbox or transmission whilst in conventional vehicles with rear wheel drive, the drive-line will also include for example the drive-shaft and the differential.
It is to be noted that these problems are potentially more pronounced in the Applicant's engine which typically runs in a highly stratified mode (ie: has excess air). The fuelling rate of the engine may be increased very rapidly (ie: can be increased as such in one cycle) in response to rapid changes in engine load demand and hence engine torque can rise very rapidly. Accordingly, there is a need for an alternative means of controlling the engine idle speed during vehicle motion so as to improve the tip-in and tip-out feel of the motor vehicle.
It is therefore an object of the present invention to provide improved control of the idle speed of an internal combustion engine which at least substantially avoids one or more of the above noted problems.